Abstract:

According to a survey conducted by the Federal Highway Administration (FHWA), 24 % of existing bridges are classified as structurally deficient. Since prestressed concrete was adopted as a method of construction in the United States in 1950, many existing bridges are approaching the latter portion of their design lives. Assessment of these existing bridges is becoming increasingly important due to the high cost of bridge replacement in combination with stringent fiscal constraints to which transportation agencies are bound.

This study was undertaken in order to provide further data on full-scale girders that have been in service for a number of years. Three precast, pretensioned, box girders, with a span of 70 feet (21.33 m), were tested. The girders had been in service for 20 years. The major objectives of the first part of the study were to examine strength and serviceability parameters and compare these value with existing code equations. The ultimate flexural strength, cracking strength, level of prestress, ductility and corrosion were investigated, since these factors profoundly affect the behavior of the girders. There was good agreement between code predicted and measured ultimate load. However both ductility and prestress losses differed significantly from code predictions. Explanations for these differences are presented.

The second part of the study involved the testing of a strand repair system and assessment of the repaired girder strength under design fatigue loads and periodically applied static overloads. Recent research has indicated that overloads can cause a significant reduction in fatigue life. The principal cause of strand damage is side impact due to overweight vehicles and corrosion. A strand repair method was developed in order to provide a cost effective and efficient alternative to relatively expensive girder replacement. The test results indicated that the repair method implemented had adequate strength but may be susceptible to fatigue damage. Static overloading in combination with design fatigue loads produced significant strand stress range increases (Abstract by authors).